Conventional ultrasonic flaw detectors are presently used to detect flaws produced during the manufacture or use of many types of metals and ceramic components, including, for example, testing of structural members of vehicles for cracks and testing of newly manufactured objects such as ingots and welded steel components for inhomogeneities. The oldest and most widely used ultrasonic flaw detection technique is a pulsed echo type wherein short waves of radio frequency ultrasound are transmitted into a test object with the echoes from inhomogeneities in the object being displayed oscillographically. The time of occurrence and amplitude of these echoes can be related respectively to the location and magnitude of the sound reflected.
Such pulse echo flaw detectors have also been used heretofore, for example, for scanning and imaging organs in the body, particularly the brain. Such scanners have a number of advantages over the other types of scanners utilizing, for example, X-rays, including decreased danger relative to X-ray exposure, but heretofore such scanners have not been capable of the resolution afforded by X-ray systems. However, prior ultrasound systems have not been capable of minimizing exposure danger to the extent desired since sufficiently small intensities have not heretofore been possible while achieving the desired end. In addition, resolution and reverberation problems have been encountered.
The limitations of pulse echo flaw detectors arise from the fact that it is necessary to wait until the most distant echo has returned before transmitting another pulse in order to avoid problems of range ambiguity. Furthermore, to obtain fine range resolutions it is necessary to transmit a correspondingly narrow burst of r.f. energy. The relative parameter may be described as follows: ##EQU1## Thus for pulse echo systems, the ratio of peak-to-average transmitted power has to be at least as large as the ratio of the maximum to the desired range resolution. This ratio will usually be on the order of 10.sup.2 or more in practical systems. Since transducers are limited in the peak power they can handle by electrical breakdown effects, the large peak to average power ratio required can limit the maximum ratio of range to resolution that can be obtained by pulse echo systems.
The other major problem faced by pulse echo flaw detectors is the fact that strongly sound absorbing material makes it necessary to use the largest possible average transmitted power if the returning echoes are to be larger than the thermal receiver noise. Since ultrasound transducers are strongly limited in average power handling capability by overheating, this limits the range of pulse echo systems when used in strongly sound absorbing or scattering materials.
Since the thermal noise power of amplifiers is proportional to bandwidth, it can be shown that the ratio of the signal-to-noise power at the output of a flaw detection system to that of the output of the echo receiving amplifier is given by the expression ##EQU2## wherein B.sub.in and B.sub.out are respectively the bandwidths of the input and output of the system and m is the mark-time ratio.
In pulse echo systems currently in use, the bandwidth of the signal received at the receiver and the bandwidth of the output signal emerging from the detector are approximately the same, thus these systems are not able to improve the signal-to-noise ratio of the received echo, and the received echo must therefore be much larger than the thermal noise of the echo of the amplifier.
Time averaging techniques have heretofore been utilized to improve the input signal to noise ratio of flaw detection systems by integrating the echo signals over relatively long time periods. This, however, effectively restricts the output bandwidth.
In at least one known time integration system, a lock-in amplifier is utilized whereas in another known system the echo signal is digitized and a digital computer is then utilized. Both of these prior art systems, however, require transmission of short bursts of r.f. energy and therefore require high peak-to-average power ratios.
Thus, while the flaw detection systems have heretofore been suggested and/or utilized, these systems have not provided completely satisfactory flaw detection and more particularly have not proved to be entirely suitable in providing a system having excellent sensitivity and high resolution while minimizing danger of exposure to, or by limiting, needed intensities.